Semiconductor devices



May 10, 1960 R. N. HALL 2,936,256

SEMICONDUCTOR DEVICES Filed June l, 1954 United States Patent O2,936,256 SEMICONDUCTOR DEVICES Robert N. Hall, Schenectady, N .Y.,assignor to General Electric Company, a corporation of New YorkApplication June 1, 1954, Serial No. 433,450 Claims. (Cl. 148-33) Myinvention relates to semiconductor devices, and, more particularly, tosuchzdevices of the broad area P-N junction type.

Semiconductors, such as germanium and silicon, have been conventionallyclassied as either positive (P-type) or negative (N-type), dependingprimarily upon the type and sign of their predominant electricalconduction carriers. Whether a particular semiconductor body exhibitsP-type or N-type characteristics is dependent lprimarily upon thepredominant type of significant impurity element or activator presentwithin the semiconductor body. Some impurity activator elements such asarsenic, antimony and phosphorus, known as donors, function to furnishadditional electrons to the semiconductor body, producing an N-typesemiconductor with an 'excess of electrons. Other impurity activatorelements, such as aluminum, gallium and indium, known as acceptors,function toremove electrons from the semiconductor lattice structure andform a P-type semiconductor body with a deciency of electrons or anexcess of positive holes, which comprise positive conduction carriers.P-N junction semiconductor devices, such as those disclosed in`application Serial No. 187,490, William C. Dunlap, tiledV September 29,1950, now abandoned, and assigned to the same assignee as the presentinvention, have a zone of P-type semiconductor material adjoining a zoneof N-type semiconductor material to form an internal space chargebarrier having a relatively broad area junction as distinguished fromthe point contact type of semiconductor device.' P-N junctions of thisnature possess marked rectifying properties as well as therxnoelectricand photoelectric properties. Two or more P-N junctions maybeincorporated within a single semiconductor device in order to producea threeor four-terminal amplifying device known as a transistor. Therectifying properties of such a `PN junction are such that when thesemiconductor body incorporating the junction is connected to -a-voltagewith a certain polarity there is a free flow ofI conduction carriersthrough the body. This polarity is known as the forward direction offlow. When the semiconductor body is connected to a voltage having theopposite polarity, the internal space charge barrier presenta a highimpedance to the flow of conduction carriers and there is littleappreciable flow of current through the semiconductor body, providingthe applied voltage is. maintained below the critical or breakdownvoltage, atywhich the internal space charge barrier is ruptured.

P-N junctions within semiconductor bodies may be formed in a number ofways. When a semiconductor device` is of the extended area, P-N junctiontype, one method of forming a P-N junction within Ithe semiconductorbody may be an alloying process in which a donor oracceptor activatormaterial s brought into contact at high temperature with one surface ofa semiconductor body, `usually a wafer of small thickness. The acceptoror donor impurity activator is then fused with and diffused into thesemiconductor body at high temperatures. When, under high temperatures,a conductivity-typeinducing activator material fuses to and diffuseswithin a semiconductor body of opposite conduction characteristics, theregion into which the activator impurity diffuses changes from itsorgina1 conduction characteristic type semiconductor material. Theregion between the two different typesemiconductor materials thus formsa P-N junction. .P-N junction semiconductor devices have, to a largeextent, been limited to small area units because the available acceptoractivator elements form alloying eutectics with semiconductors such asgermanium and silicon which are extremely brittle and non-durable. Thisresults `in cracking, crazing and cleavage between the alloyed surfaceand the body of the semiconductor material during cooling afteralloying.

One object, therefore, of the present invention is to provide anacceptor contact material for semiconductor devices having improvedelectrical and mechanical properties over a wide range of temperatures.

A further object of the invention is to provide extended area P-Njunction semiconductor devices having desirf able electrical andmechanical properties over a wide range of temperatures.

In accordance with one feature of this invention, there is provided asan acceptor contact material for P-N junction semiconductor devices, anacceptor alloy of gallium and aluminum. This acceptor material possessesa heretofore unavailable combination of desirable electrical andmechanical characteristics which makes possible improved extended areaP-N junction devices, as for example, rectifiers and transistors.

The novel features which are believed characteristic of the inventionare set forth inthe appended claims. The invention itself,.together withfurther objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawing, in which:

Fig. 1 is a perspective view of a P-N junction semiconductor deviceembodying the invention;

of Fig. 1;

Fig. 3 is an electrical of Fig'. l;

Fig. 4 is a cross-section view of an alternative embodiment for thedevice o`f`Fig. 1; and

Fig. 5 is a vertical cross-section view of a three-terminal transistorconstructed according to this invention.

Extended area P-N junction semiconductor units, with which thisinvention is concerned, may be produced in a number of ways. One commonmethod of production is by the fusion and diifusion of an impurityactivator element to and within the surface of a thin wafer ofsemiconductor material as, for example geranium or silicon. According toone feature of this method of production, a thin wafer of crystalline,N-type germanium or silicon isprepared according to methods which arewell known and will not be herein discussed. Such a crystalline, N-typesemiconductor wafer is then brought characteristics graph of the deviceinto surface contact with an impurity activator elementV common manner,however, is thermal alloying and diffuj sion. In such a process a smallamount of impurity activator material may be placed upon the surface ofa semiconductor wafer into which a P-N junction is to be induced, andheld thereon with pressure while the two elements and subjected to ahigh temperature suicient to cause partial alloying of the semiconductorwith the activator impurity. After proper alloying and diffusion havetakenV place, the semiconductor body may be allowed to cool and a lowresistance contact soldered or r otherwise connected to the exteriorsurface `of the semi- 4 conductor body. Alternatively, the impurityactivator material mayserve as a solder to bond a low resistance.contact'to the semiconductor body in a single step. In producingajsemiconductor device of this type, however, -many fdiiiculties arise.Presently available acceptor alfloys' for kgermanium and silicon such asaluminum, gal- `lium, and indium-are not mechanically ideal for formingbroad area contacts with -those semiconductor materials. .Gallium isaliquid at room temperatures and is not suitable. indium has a lowmelting point (155 C.) and is -limited in use to low temperatures. Thethermal coeiiicientsv of expansion of the eutectics formed by aluminumwith-germanium `and silicon are markedly different from `the thermalcoeflicients of expansion of lgermanium and silicon. Such differencescause thermal strains which canrdamage or ruin a broad area contactbetween an activator and a semiconductor body. When laluminum 'isused-to form abroad area P-,N junction withl germaniumv or silicon abrittle junction is formed and during coolingafter alloying, the brittlealloy between the activator element and lthe silicon semiconductor bodyWillordinarily crack Vor craze and render the serniconductor deviceuseless. An additional factor makes available acceptor elementsunsuitable for forming, extended area contacts with silicon. Siliconisrnot lreadily Wet by alloying materials. Pure indium does not wetsilicon readily. Additions of other metals to indium to cause wettingtends to destroy the acceptor characteris-l tics of indium. Hence,neither aluminum, gallium or indium maybe used as an acceptor activatorcontact for anr extended area silicon Arectifier or semiconductordevice. l

I have found that, although gallium andaluminum'are individually notsuited as broad area acceptor activator contacts for germanium andsilicon semiconductor bodies, a mixture of gallium in aluminum 4resultsin an alloy which is sutiiciently solid at normal semiconductor deviceoperating temperatures to provide the required mechanical strength. Thisalloy, when fused to a semiconductor body to form a P-N junctiontherewith, or merely to supply positive conduction carriers thereto,is.. nevertheless, able to withstand large temperature variationswithout shearing `from the semiconductor, cracking or crazing. Thereisprovided, therefore, an acceptor activator material for semiconductordevices which lhas desirable electrical and mechanical characteristicsand which readily wets silicon. Therefore, while this activator materialmay be used effectively `with both germanium and silicon, it isparticularly useful with silicon, for which no other acceptor activatorsuitable for extended area contacts at any temperature has beenheretofore available.

In a preferred form of my invention the diffusion of the acceptoractivator alloy material is combined with attaching of a low resistancecontact to the semiconductor device by using the jacceptoractivatormaterial as a vsolder between the two. This is accomplished by heating asemiconductor wafer, a portion of acceptor activator material, and a lowresistance contact together in sandwich fashion at a temperature and fora period of time su'icient to cause the desired degree of impuritypenetration and bonding.

lIn Fig. 1 there is shown one type of P-N junction semiconductor unitmade in accordance with the present invention. This device, denominatedgenerally as .10, is Ian extended area Ysemiconductor rectifier,comprising an N-type semiconductor wafer 11 and two lowresistanceelectrodes 12 and 16, which may conveniently be nickel orfernico in electrical contact with opposite major Ifaces of wafer 11.Low resistance electrode 12 is fastened to wafer 11v by the hereinbeforedescribed ac- Y ceptor lactivatorsolder 13 comprising an alloy of alu-N-type semiconductor wafer 11 causing a surface adjacent region 14 ofsemiconductor wafer 11 to be transformed into P-type semiconductor. Ygion between surface adjacent region 14 and the body of wafer 11 forms abroad area P-N junction 15. Acceptor -activator solder 13 is ari-.alloyof gallium in aluin the range of from to 35% by weight of gallium, andis preferably composed @of approximately gallium and 70% aluminum. Thisacceptor activator alloy acts as a very satisfactory source ofvacceptoractivator Iatoms to transform surface adjacent region 14 of N-typesemiconductor wafer-11 into P-type semiconductor and also forms asufficiently ductile alloy with semiconductor wafer 11Y which allows theWafer to expand and contract with temperature changes Without causingcracking, shearing, or crazing between solder 13 and Wafer 12. ,Lowresistance electrode 16 is connected with semiconductor Wafer 11 bymeans of solder layer 17 which may be an alloy of Va donor activatormaterial which has good thermo-mechanical properties. This alloy mayconveniently be an alloy ofra small percentage of a donor activatorelement such as arsenic, antimony and phosphorus, the remainder beingindium,

mimlm and sallium which :is-.fused t0 and dittused'within 'i5 asdescribed in application Serial No. 410,609, A.lohn yS. Baby, filedFebruary 16, 1954, and assigned torthesame assignee as the presentinvention. For this purpose `a mixture of v10% to 30% by weight of Yadonor activator element in indium may be conveniently used. Since thesegregation constant of indium in germanium and silicon, that is, theratio of solid ,solubility of indium in germanium and silicon to thepercentage of 'indium' in the liquid phase upon alloying, is verymuchfsmaller than the segregation constant of the donor elements ingermanium or silicon, the electrical characteristics of the junctionformed by the indium-donor element .alloy with the semiconductor waferare dominated by the -donor element. On the other hand, since there ismuch more indium present in the alloy,.it possesses essentially the samemechanical properties as `pure indium, that-Vis,

the alloy is suiiciently ductible to withstand temperature stresswithout cracking or crazing and forms a contact having Vthe necessaryductibility for largei area contact with semiconductor wafer 11.Additionally, this alloyH` readily wets germanium and silicon whenused'asa solder between low resistance electrode 16 and waferfll.v

ness may, however, vary from very thick wafers tothe thinnest possibledimensioncommensurate withmechanical strength, depending upon theultimate Vilse of the device. 'Ihe length and width of the wafer are'notcritical but may conveniently be approximately 1A". The forming of P-Njunction-15 within the rectifier 10 of Fig. 2 `and the simultaneoussoldering of electrode 12 to silicon wafer 11`by means of acceptoractivator alloy 13 on one hand, and the soldering of electrodexlt toVwafer 11 by means of donor activator alloy `17 0n the other hand, may beaccomplished in `one heating operation, or may be performed in twoindependentope'ra" tions.` According to a preferred procedure bothoperations rnay be performed simultaneously. Thisfmay be accomplished byplacing silicon Wafer r11, electrodes 12 and 16 which are preferably ofYnicke'l,an"d thin wafers of donor alloy 17 and acceptor alloy 12together` The boundary re-Y Wafer 12 mayl vconveniently `be :a thinfurnace or other heating chamber. Heating is conducted at a temperatureand for a sucient length of time to cause acceptor alloy 12 to wet, andpartially diffuse into silicon wafer 11 so as to form P-N junction 15,but not completely diffuse throughout the body of silicon Wafer 11.Complete dilusion would change the entire body of wafer 11 into P-typesilicon and no P-N junction would be formed. This temperature may varysomewhat but should be above about 600 C., the temperature at which thesilicon wafer 11 rst begins to be wet by acceptor alloy 12, and belowabout 900 C., the temperature at which excessive alloying and diffusionof acceptor alloy 12 into silicon wafer 11 begins to occur. The time ofheating may vary somewhat depending upon the thickness of silicon wafer11. Using a silicon wafer of approximately 0.020" thick, a suitable P-Njunction bas been found to be formed by a heating at 700 C. of -f-rom 1Ato 2 minutes.

A rectifier constructed laccording to the above method will then besuitable as a rectifying semiconductor device over a large range oftemperatures.

In Fig. 3 there is shown the rectiication characteristics of a broadarea silicon rectifier made according to this invention using a 20 ohmcentimeter N-type silicon wafer 0.020" thick. As may be seen from thecurve of Fig. 3, rectiiier 10 will pass high currents up to severalthousands of milliamperes for voltages of less than 10 volts whenconnected in the forward or easyow direction. When polarity is reversed,the leakage current of rectifier 10 is extremely low, being less than 30milliamperes for reverse voltages up to 200 volts. The rectifyingcharacteristics of rectifier 10 as shown in Fig. 3 vary extremely littleover temperature ranges from liquid nitrogen temperature to 200 C. Theforward resistance of this rectiiier may be greatly reduced by using athinner wafer of lower resistivity.

In Fig. 4 there is shown another embodiment of the invention. TheVrectifier formed in Fig. 3 diifers from that of Fig. 2 only in thatnickel electrode 13 has been dispensed with, and lead 18 is connecteddirectly to a body of gallium-aluminum acceptor activator material 19which is fused to and diffused within N-type semiconductor wafer 11a.

Another embodiment of my invention is illustrated in Fig. 5, where athree electrode transistor is shown formed from a wafer of N-typesemiconductor which is preferably cut from a single crystal of N-typematerial. A base electrode 20 which may be any donor activator material,as, for instance arsenic or antimony or an alloy of indium and a donoractivator, is fused to and Within one major face of semiconductor wafer11. Terminal lead 21 is soldered to electrode 20. An emitter electrode13e comprising the improved acceptor activator alloy of aluminum andgallium, from 25 to 35%, and preferably about 30%, gallium in aluminum,is fused to and within the same major face of semiconductor wafer 11e,and heated for a suicient time and at a suicient temperature to causealloying therewith. 'Ihe diffusion of the activator alloy withinsemiconductor wafer 11c transforms surface adjacent region 14c withinsemiconductor wafer 11e into P-type semiconductor and a P-N junction eis formed between the P-type surface-adjacent region 14e and the body ofN-type semiconductor wafer 11c. Terminal lead l8r,` is connected toemitter electrode 13a'. A collector electrode 22 to which terminal 23 isconnected is furnished for the transistor of Fig. 5 by soldering a goodconductor material which may, for instance, be fernico, to the oppositemajor face of wafer 11c with the acceptor alloy of aluminum and galliumas described hereinbefore. This alloy may be from 25 to 35 gallium inaluminum, and is preferably approximately 30% gallium, the remainderbeing aluminum. Heating to form the solder is conducted at a temperatureand for a time suicient to cause the diffusion of the acceptor alloywithin the N-type wafer 11C to cause the formation of a surface adjacentregion 24 and a P-N junction 2S.

It is obvious that, although my invention has been described inconnection with specific embodiments, many modiiications may be madewithout departing from the spirit of the invention. It is to beunderstood, therefore, that I intend by the appended claims to cover allsuch modifications as fall within the true spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent ofthe UnitedStates is:

1. A semiconductor device comprising a crystalline semiconductor bodyselected from the group consisting of germanium and silicon and anacceptor activator material fused to onesurface thereof, said acceptoractivator material consisting essentially of an alloy of 25 to 35 weightpercent of gallium, the remainder being aluminum.

2. A semiconductor device comprising a crystalline semiconductor bodyselected from the group consisting of germanium and silicon and anacceptor activator material fused to and diffused within a surfaceadjacent region of said semiconductor body and forming an extended areacontact therewith, said acceptor activator material consistingessentially of an alloy of 25 to 35 weight percent of gallium, theremainder being aluminum.

3. A semiconductor device comprising a crystalline N- type semiconductorbody selected from the group consisting of germanium and silicon and anyacceptor activator material fused to and diffused within a surfaceadjacent region of said semiconductor body to form a P-N junctiontherein, said acceptor activator material comprising A, from 25 to 35%gallium, the remainder being aluminum.

4. The device of claim 3 wherein the semiconductor body consists ofN-type silicon.

5. An asymmetrically conducting device comprising a crystalline wafer ofN-type silicon, an acceptor activator material comprising from 25 to 35%gallium, the remainder being aluminum, fused to and dilfused within onemajor surface of said wafer to form a P-N junction,

therein, a donor activator alloy comprising from l0 to 30% by weight ofarsenic, the remainder being aluminum, fused to and diffused within theopposite major face of said wafer.

ReferencesCited in the iile of this patent UNITED STATES PATENTS2,369,354 Kempf Feb. 13, 1945 2,579,369 DaWe Dec. 18, 1951 2,694,024Bond Nov. 9, 1954 2,701,326 Pfann Feb. l, 1955 2,705,767 Hall Apr. 5,1955 2,725,315 Fuller Nov. 29, 1955

1. A SEMICONDUCTOR DEVICE COMPRISING A CRYSTALLINE SEMICONDUCTOR BODYSELECTED FROM THE GROUP CONSISTING OF GERAMANUIM AND SILICON AND ANACCEPTOR ACTIVATOR MATERIAL FUSED TO ONE SURFACE THREOF, SAID ACCEPTORACTIVATOR MATERIAL CONSISTING ESSENTIALLY OF AN ALLOY OF 25 TO 35 WEIGHTPERCENT OF GALLIUM, THE REMAINDER BEING ALUMINUM.